This invention relates generally to the field of particle-based electrochemical power sources, and, more specifically, to techniques for flushing the cells thereof
Particle-based electrochemical power sources, including without limitation metal-based fuel cells, are emerging as an attractive alternative to traditional energy sources. In metal-based fuel cells, when metal anodes within the cell cavities are exposed to an electrolysis agent such as hydroxide, an electrochemical reaction takes place whereby the metal releases electrons, and a reaction product is formed, typically one or more ions or oxides of the metal. Through this process, the metal anodes are gradually consumed. The released electrons flow through a load to a cathode, where they react with a second reactant such as oxygen.
Particle-based electrochemical power sources, including without limitation metal-based fuel cells, which employ particulate anodes and which deliver reaction agents such as hydroxides to the anodes through a reaction solution pose particular challenges since, for such power sources (e.g., fuel cells) to function efficiently, it is appropriate to re-circulate the reaction solution throughout and within the particulate anodes so that the appropriate electrochemical reaction can take place. However, this recirculation and electrochemical reaction can have undesired consequences.
These unintended consequences include certain problems of particle buildup, reduction in reaction solution flow through anode bed (e.g., clogging), and precipitation of solid zinc oxide that can occur during either the discharge cycle mode of operation due to the electrochemical reaction or the standby mode of operation due to the corrosion reaction described above.
For example, as the electroactive (e.g., metal) particles in the anode beds are consumed during the electrochemical reaction, they become smaller and smaller, and can become more densely packed together. The dense packing of the small particles can lead to particle buildup, clogging and interference with, and resulting reduction of, the flow of reaction solution throughout and within the particulate anodes. This in turn can lead to the generation of insoluble reaction products, which can further reduce the flow of reaction solution throughout the cell and/or the reaction bed. The result is typically a substantially less efficient fuel cell.
In a zinc-based fuel cell, for example, clogging caused by the generation of small particles can lead to the following electrochemical reaction occurring at the anode beds:
Zn+2OHxe2x88x92xe2x86x92Zn(OH)2(s)+2exe2x88x92xe2x80x83xe2x80x83(1) 
The reaction product Zn(OH)2(s), unlike Zn(OH)42xe2x88x92, is insoluble.
A related problem is that, as electrochemical dissolution occurs, the concentration of reaction products can increase. For example, during electrochemical dissolution in a zinc-air fuel cell, zincate, Zn(OH)42xe2x88x92, in the KOH reaction solution increases. When the zincate saturation point is reached, any further electrochemical dissolution that occurs will cause zinc oxide, ZnO, to precipitate out of the KOH solution. Again, the generation of insoluble reaction products such as ZnO can further reduce the flow of reaction solution throughout the cell.
Even when the fuel cell is in a standby mode, and is not undergoing electrochemical dissolution, these problems can occur through a corrosion reaction that occurs. In the zinc fuel cell, for example, the corrosion reaction, in which zinc reacts with water, can be expressed as follows:
Zn+2H2O+2OHxe2x88x92xe2x86x92Zn(OH)42xe2x88x92+H2 xe2x80x83xe2x80x83(2) 
As can be seen, zincate is the by-product of this reaction. Consequently, as this reaction occurs, zincate will build up, and when the zincate saturation level is reached, solid zinc oxide can precipitate out and clog the anode beds as described above. Moreover, because zinc is consumed in this reaction, this reaction will also result in smaller zinc particles, which can also lead to clogging of the anode beds as described above.
To solve these and other problems, the invention provides a method of and system for flushing one or more cells or components thereof in a particle-based electrochemical power source. According to this method and system, reaction solution is delivered to and withdrawn from the one or more cells when the electrochemical power source is in a standby mode of operation.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.